Constructing Full Ionic Equations

Redox Chemistry

What is the purpose of ionic half-equations?

They show the oxidation and reduction processes separately, including the electrons transferred.

How are two ionic half-equations combined to form a full ionic equation?

Make the number of electrons in both half-equations equal, add the equations together, then cancel the electrons.

What must happen to the electrons when constructing a full ionic equation?

The same number of electrons must be lost and gained so that they cancel.

What should you do if two half-equations contain different numbers of electrons?

Multiply one or both half-equations so that they contain the same number of electrons.

What is the ionic half-equation for the oxidation of zinc?

Zn(s) → Zn²⁺(aq) + 2e⁻.

What is the ionic half-equation for the reduction of Cu²⁺?

Cu²⁺(aq) + 2e⁻ → Cu(s).

What is the full ionic equation for the reaction between Zn and Cu²⁺?

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

Why can the Zn and Cu²⁺ half-equations be added together directly?

Both involve two electrons, so the electrons cancel without multiplying either half-equation.

What is the half-equation for oxidation of Fe²⁺ to Fe³⁺?

Fe²⁺(aq) → Fe³⁺(aq) + e⁻.

What is the half-equation for reduction of chlorine?

Cl₂(g) + 2e⁻ → 2Cl⁻(aq).

How must the Fe²⁺ half-equation be changed before combining it with the chlorine half-equation?

Multiply it by 2 so that two electrons are produced.

What is the full ionic equation for the reaction between Fe²⁺ and Cl₂?

2Fe²⁺(aq) + Cl₂(g) → 2Fe³⁺(aq) + 2Cl⁻(aq).

What is the half-equation for the reduction of manganate(VII) ions in acidic solution?

MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l).

What happens to manganese in MnO₄⁻ → Mn²⁺?

Manganese is reduced from oxidation number +7 to +2 by gaining five electrons.

What is the full ionic equation between MnO₄⁻ and Fe²⁺ in acidic solution?

MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq).

Why must the Fe²⁺ half-equation be multiplied by 5 when reacting with MnO₄⁻?

Each Fe²⁺ loses one electron while MnO₄⁻ requires five electrons, so five Fe²⁺ ions are needed.

What is the half-equation for oxidation of hydrogen peroxide?

H₂O₂(aq) → 2H⁺(aq) + O₂(g) + 2e⁻.

What is the lowest common multiple of 5 and 2 electrons?

10 electrons.

How are the MnO₄⁻ and H₂O₂ half-equations adjusted before combining?

Multiply the MnO₄⁻ half-equation by 2 and the H₂O₂ half-equation by 5.

What is the full ionic equation between MnO₄⁻ and H₂O₂ in acidic solution?

2MnO₄⁻(aq) + 6H⁺(aq) + 5H₂O₂(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g).

Why are H⁺ ions cancelled when combining some half-equations?

If H⁺ occurs on both sides of the equation, equal amounts are subtracted from both sides.

What does the presence of H⁺ in a final ionic equation indicate?

The reaction takes place under acidic conditions.

Why are many redox reactions pH-sensitive?

The species involved and the products formed can depend on whether the conditions are acidic or alkaline.

What are the main steps for constructing a full ionic equation from half-equations?

Balance the electrons, multiply the half-equations if necessary, add them together, then cancel electrons and any other identical species on both sides.

What should you check after constructing a full ionic equation?

Check that both the atoms and the overall charge are balanced.

Why must charge be balanced in an ionic equation?

Charge is conserved during a chemical reaction.

How can oxidation numbers be used to balance a redox equation?

Identify the elements whose oxidation numbers change and use the size of the changes to determine the required ratio of reactants.

What does a change in oxidation number from +4 to +6 represent?

Oxidation involving a two-electron change.

What does a change in oxidation number from +1 to 0 represent?

Reduction involving a one-electron change.

If one element undergoes a two-electron change and another undergoes a one-electron change, what ratio is required?

1 of the two-electron change to 2 of the one-electron change.

How are oxygen atoms commonly balanced in acidic redox equations?

Add H₂O to the appropriate side.

How are hydrogen atoms commonly balanced in acidic redox equations?

Add H⁺ ions to the appropriate side.

What is the final check when balancing a redox equation using oxidation numbers?

Check that the total charge is the same on both sides.

How does sulfur change in SO₂ → SO₄²⁻?

Sulfur increases from +4 to +6, so it is oxidised.

How does silver change in Ag⁺ → Ag?

Silver decreases from +1 to 0, so it is reduced.

What is the balanced equation for SO₂ reacting with Ag⁺ in acidic aqueous conditions?

SO₂(g) + 2H₂O(l) + 2Ag⁺(aq) → SO₄²⁻(aq) + 4H⁺(aq) + 2Ag(s).

Why are two Ag⁺ ions required for each SO₂ molecule in this reaction?

Sulfur loses the equivalent of two electrons while each Ag⁺ gains one electron.

How does Fe²⁺ change when it forms Fe³⁺?

Iron increases from +2 to +3, so it is oxidised by a one-electron change.

How does chlorine change in ClO₃⁻ → Cl⁻?

Chlorine decreases from +5 to −1, so it is reduced by a six-electron change.

What ratio of Fe²⁺ to ClO₃⁻ is required when Fe²⁺ is oxidised by ClO₃⁻?

6:1, because six Fe²⁺ ions each lose one electron while one chlorine gains the equivalent of six electrons.

What is the balanced ionic equation for Fe²⁺ reacting with ClO₃⁻ in acidic solution?

6Fe²⁺(aq) + ClO₃⁻(aq) + 6H⁺(aq) → 6Fe³⁺(aq) + Cl⁻(aq) + 3H₂O(l).

What should you identify first when balancing an equation using oxidation numbers?

The elements whose oxidation numbers have changed.

Why are oxidation-number changes compared when balancing redox equations?

The total number of electrons lost during oxidation must equal the total number gained during reduction.

What does multiplying a half-equation do?

It multiplies every species and electron in that half-equation by the same factor.

Can you change the chemical formula of a species when balancing a redox equation?

No. Only coefficients in front of species can be changed.

What happens to electrons in the final full ionic equation?

They cancel completely and do not appear in the final equation.